1. Field of the Invention
This invention relates generally to the use of myocardial electrical impedance (MEI) measurements for evaluating and diagnosing the physiologic and biochemical state of the human myocardial muscle, particularly during and after surgical procedures.
2. Description of the Related Art
Myocardial electrical impedance has been studied for over 125 years. Besides our work, no past research has involved human subjects. Myocardial electrical impedance holds promise in detecting and assessing various disease states of human heart tissue. Through animal and tissue experiments, MEI has been shown to correlate with the vitality of the myocardial tissue. Myocardial electrical impedance changes predictably with regional and global ischemia [1-3], edema [4], pathologic tissue ultrastructural changes (4), ATP depletion and lactate accumulation [1]. Methods and apparatus for measuring MEI are described in U.S. Pat. No. 5,454,377 which is herein incorporated by reference.
Recently researchers have demonstrated that MEI can reliably detect humoral rejection episodes following heart transplantation [5-7]. Cardiac tissue rejection results in global myocardial ischemia and other conditions that have previously been shown in animals to affect MEI. Currently, the condition of the transplanted cardiac tissue is monitored by routine histopathologic biopsies taken from the myocardium. The results from this procedure often take several days to obtain whereas MEI can be measured in a matter of seconds and can be measured more often, more safely and far less expensively than tissue biopsies. The benefit of using MEI rather than tissue biopsies is that tissue rejection could be diagnosed and treated earlier which will result in better preservation of the transplanted heart.
Myocardial electrical impedance has been used to compare myocardial preservation methods [3], measure the revivability of the heart [4] and to gauge tissue protection by beta-blockade from reversible ischemic damage [1]. It has been shown in animal experiments that MEI responds both to ischemia and reperfusion of the tissue [8]. Myocardial electrical impedance can therefore detect severe but reversible ischemic injury. The variable has been shown to assess the effects of cardioplegia and determine the need for and type of resuscitation following coronary artery bypass surgery. Myocardial electrical impedance can also assess the effects of tissue preconditioning, an important component of coronary artery bypass graft (CABG) surgery. Because MEI is anisotropic, where its absolute value depends on the orientation of the electrodes with the muscle fibers at any point in time, the parameter can also be used to determine the anatomic and physiologic orientation of the fibers. The physiologic orientation of the fibers changes with disease.
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